Why Ladies-Only Species Don’t Need Men

[The Spartan]

Wired.com

How all-female species avoid the shrinkage of their gene pool is among the animal kingdom’s great mysteries. Now biologists think they’ve discovered the trick.

According to a study published Sunday in Nature, egg-producing cells in a Aspidoscelis tellesata, a ladies-only species of whiptail lizard, contain double the standard genetic complement. They pick the healthiest set of chromosomes, preventing the loss of vital variation.

“There’s an absence of sperm, and genetic information is never provided by another source. Anything that’s lost is lost for good,” said Peter Baumann, a University of Kansas cell biologist. “If there’s a way to prevent the loss, then how is that accomplished? That’s what our paper explains.”

In all animal cells, genes are contained in DNA packages called chromosomes. Cells have two copies of each chromosome, and thus of each gene: In sexually reproducing species, one copy comes from mom, and the other from dad.

During reproduction, germ cells duplicate their chromosome set, then go through two rounds of cell division. The result is a sperm or egg cell with one copy of each chromosome. Some genetic variation is lost in the process, but it doesn’t matter, since sperm and egg soon fuse. Losses are offset by the mixing of their union.

In creatures that develop from unfertilized eggs — a process called parthenogenesis, found in A. tellesata, plus about 70 other fish, reptile and bird species — both chromosome copies come from mom. Lost genetic variation is unrecoverable, and ought to accumulate over many generations, eventually producing an animal unable to survive.

But as Baumann’s group shows, A. tesselata germ cells start with four sets of chromosome copies, not two. When the cells finish dividing, the resulting eggs contain a standard set, one that’s assembled, they found, from two loss-free copies.

whiptail_gametes

“This is an elegant mechanism,” said Baumann. And though they don’t yet know how A. tesselata’s germ cells evolved this trick, “you can imagine that it happens by a relatively simple change.” That could explain why parthenogenesis reproduction has emerged in so many species.

Whether parthenogenesis is an evolutionary aberration or viable strategy remains debated. Even if these species maintain their gene pool, they still lack the genetic mixing that gives sexual reproducers a steady flow of new adaptations.

According to Baumann, A. tesselata is lucky: It appears to be descended from a union of two related species, giving it a hybrid vigor. As for populations lacking built-in durability, he said asexual reproduction may be a useful short-term strategy. It could maintain lineages through periods of isolation, with species reverting to sexual reproduction when suitable partners were available.

However, Baumann cautioned against assuming that all parthenogenic species used the mechanisms seen in A. tesselata.

“I think it’s going to be widespread, but nature often surprises us<” he said. “We think we know how something works, then find out that nature came up with many ways of doing it.”

I'm afraid I can't add anything insightful, but it was interesting enough that I wanted to share with the board. In fact, I hadn't heard that there was even such a thing.

[TabascoOne]

Not much to add either. I knew bees did something like this, but had no idea it was so widespread.

I wonder though about the reverting to sexual reproduction part. After a long period of asexual reproduction, I would think there wouldn't be nearly much genetic diversity to be had. Wouldn't that limit the value of the ability to switch over. Though I suppose anything is better than nothing in that instance.

[Broomstick]

Bees are not parthenogenic. Queens mate with males, then store the sperm the rest of their lives. The worker bees and queens are products of sexual reproduction, only the drones have no fathers. The technical term is "haplodiploidy"

Parthenogenetic species that truly have no males are a different type of reproductive system.

[Anguirus]

As I understand it, parthenogenesis pops up somewhat often in vertebrates but has self-evidently never stuck around in major lineages (70 species all spread out is not very many). It's not super-hard to evolve it in circumstances when finding mates is difficult, but it's not as generally advantageous as sexual reproduction. I don't know if parthenogenic species can evolve back to sexual reproduction or if they ar just a bunch of truncated lineages over geological time.

Oh, and whiptail lizards are a great way to fluster anti-gay crusaders and intelligent designers. They hump each other, and this has been shown to increase the viability of both females' clutches!

[Mayabird]

Mostly truncated lineages over geological time. Whiptail lizards have had the same general environment for millions of years so they don't need the genetic recombinations and mixing so much. Also, they actually must have hot lizard lesbian sex (for certain definitions of "hot lizard" and please don't share) to lay their eggs. It gets the hormones flowing, or something. It's been a while since I had that class and I think the prof was hamming it up to try to make the homophobic students squirm.

Rotifiers are the exception since they pick up DNA from wherever and stick it in their genome for possible use whenever.

[TabascoOne]

Bees are not parthenogenic. Queens mate with males, then store the sperm the rest of their lives. The worker bees and queens are products of sexual reproduction, only the drones have no fathers. The technical term is "haplodiploidy"

Parthenogenetic species that truly have no males are a different type of reproductive system.

Thanks for the correction, obviously my undergrad biology is a little rustier than I thought.

And that's about what I would expect as far as long term consequences. It would need a very stable environment, and especially a disease free environment with such a lack of genetic diversity. I'm pretty surprised that some opportunistic microbe would have come along and done them in by now.

[Alyrium Denryle]

Bees are not parthenogenic. Queens mate with males, then store the sperm the rest of their lives. The worker bees and queens are products of sexual reproduction, only the drones have no fathers. The technical term is "haplodiploidy"

Parthenogenetic species that truly have no males are a different type of reproductive system.

Thanks for the correction, obviously my undergrad biology is a little rustier than I thought.

And that's about what I would expect as far as long term consequences. It would need a very stable environment, and especially a disease free environment with such a lack of genetic diversity. I'm pretty surprised that some opportunistic microbe would have come along and done them in by now.

They live in the SW united states. Pretty damn stable. 30 thousand years ago it was a pinion-juniper forest, but you will find those in the northern part of their range. So not a whole lot of variation.